River water temperature in the United Kingdom Changes over the 20th century and possible changes over the 21st century

TLDR: In this article, a review provides a new perspective on the topic by evaluating changes in river water temperature for the UK over the 20th century and possible changes over the 21st century.

Abstract: Change in river water temperature has important consequences for the environment and people. This review provides a new perspective on the topic by evaluating changes in river water temperature for the UK over the 20th century and possible changes over the 21st century. There is limited knowledge of space-time variability in, and controls on, river temperature at the region scale and beyond over the 20th century. There is historical evidence that UK river temperature has increased in the latter part of the 20th century, but low agreement on the attribution of changes to climatic warming because river temperature is a complex, dynamic response to climate and hydrological patterns moderated by basin properties and anthropogenic impacts. Literature is scarce to evaluate changes to UK river temperature in the 21st century, but it appears as likely as not that UK river temperature will increase in the future. However, there are a number of interlinked sources of uncertainty (related to observations, scenarios,...

Figures

Figure 2. Conceptual model of multivariate, nested controls on river temperature that interact to produce site-specific dynamics. Green text indicates controls that moderate dynamics and sensitivity to climate. Red text indicates controls that enhance dynamics and sensitivity to climate.

Figure 4. Length of records (years) for sites in the Environment Agency of England and Wales’ Freshwater Temperature Archive.

Figure 10. Box plots of (a) air-water temperature sensitivity index (SI) strength, (b) mean basin permeability, (c) basin area and (d) mean basin elevation for river temperature stations in each region. See Figure 7 for spatial distribution of stations by regions. SI values close to 0 indicated greater river temperature sensitivity to air temperature than those closer to 1.

Figure 3. River water temperature at Trowbridge on Biss (BFI ¼ 0.17) and Amesbury on Avon (BFI ¼ 0.97). (a) Diurnal regimes for hottest seven days in 2007. (b) Diurnal regimes for coldest seven days in 2007. (c) Annual regimes for 2007. (d) Location of river temperature gauging stations within the UK. BFI denotes the Base Flow Index, which is a measure of the proportion of runoff derived from stored sources. Higher BFI values represent the more permeable catchment, i.e. higher water storage in bedrock, superficial deposits and soils.

Figure 7. Maps of (A) mean, (B) minimum, (C) maximum and (D) range in river water temperature for 2832 sites with >250 samples between 1990 and 2007 from Environment Agency of England and Wales’ Freshwater Temperature Archive. For each variable, the England and Wales mean (m) and standard deviation (s) are provided as text; and each site is colour-coded by z-scores (blue for negative site z-values, below the England and Wales average; red for positive site z-values, above the average). Source: After Cooper (2010); Orr et al. (2010).

Figure 1. A schematic representation of the energy and hydrological fluxes controlling river water temperature. Source: After Hannah et al. (2008b)

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University of Birmingham
River water temperature in the United Kingdom
Hannah, David M.; Garner, Grace
DOI:
10.1177/0309133314550669
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Creative Commons: Attribution (CC BY)
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Citation for published version (Harvard):
Hannah, DM & Garner, G 2015, 'River water temperature in the United Kingdom: Changes over the 20th century
and possible changes over the 21st century', Progress in Physical Geography, vol. 39, no. 1, pp. 68-92.
https://doi.org/10.1177/0309133314550669
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Article
River water temperature in the
United Kingdom: Changes over
the 20th century and possible
changes over the 21st century
David M. Hannah and Grace Garner
University of Birmingham, UK
Abstract
Change in river water temperature has important consequences for the environment and people. This
review provides a new perspective on the topic by evaluating changes in river water temperature for the UK
over the 20th century and possible changes over the 21st century. There is limited knowledge of space-time
variability in, and controls on, river temperature at the region scale and beyond over the 20th century. There
is historical evidence that UK river temperature has increased in the latter part of the 20th century, but low
agreement on the attribution of changes to climatic warming because river temperature is a complex,
dynamic response to climate and hydrological patterns moderated by basin properties and anthropogenic
impacts. Literature is scarce to evaluate changes to UK river temperature in the 21st century, but it appears
as likely as not that UK river temperature will increase in the future. However, there are a number of inter-
linked sources of uncertainty (related to observations, scenarios, process interactions and feedback) that
make estimating direction and rate of temperature change for rivers across the UK with confidence very chal-
lenging. Priority knowledge gaps are identified that must be addressed to improve understanding of past, con-
temporary and future river temperature change.
Keywords
adaptation, climate change, controls and processes, review, river/stream temperature, spatial and temporal
heterogeneity, thermal dynamics, UK, water temperature
I Introduction
Water temperature is recognized increasingly
by scientists, environment managers and regula-
tors as an important and highly sensitive ‘mas-
ter’ variable of water quality (Hannah et al.,
2008b). Temperature directly influences: distri-
bution (e.g. Boisneau et al., 2008), predator-
prey interactions (e.g. Boscarino et al., 2007),
survival (e.g. Wehrly et al., 2007), growth rates
(e.g. Imholt et al., 2010, 2011; Jensen, 2003),
timing of life history events (e.g. Harper and
Peckarsky, 2006) and metabolism (e.g. Alvarez
and Nicieza, 2005) of aquatic organisms in river
systems. Indirectly, temperature controls in-
stream processes such as rates of production,
nutrient consumption and thus food availability,
Corresponding author:
David M. Hannah, School of Geography, Earth and
Environmental Sciences, University of Birmingham,
Edgbaston, Birmingham B15 2TT, UK.
Email: d.m.hannah@bham.ac.uk
Progress in Physical Geography
2015, Vol. 39(1) 68–92
ª The Author(s) 2015
Reprints and permission:
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DOI: 10.1177/0309133314550669
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decomposition (e.g. Ormerod, 2009) and dis-
solved oxygen concentration (e.g. Sand-Jensen
and Pedersen, 2005), which influence ecologi-
cal processes further. In addition, water tem-
perature is of economic importance for electric
power (e.g. Foerster and Lillestam, 2010; van
Vliet et al., 2013b), drinking water production
(e.g. Ramaker et al., 2005) and fisheries (e.g.
Bartholow, 1991; Ficke et al., 2007). Hence,
there are clear ecological and socio-economic
benefits (Poole and Berman, 2001) to be
accrued from: (1) understanding the sensitivity
of river temperature to climate and other drivers
of change using observations for the 20th cen-
tury; and (2) assessing possible future river tem-
perature changes in the 21st century to inform
management and adaptation strategies (Wilby
et al., 2010). As a consequence of the increas-
ingly recognized importance of river tempera-
ture, there has been an upsurge in research on
this water quality variable (Hannah et al.,
2008b).
There are four comprehensive reviews of the
river temperature literature. In chronological
order, Smith (1972) considers the physical pro-
cesses driving river temperature variability in
near-natural systems and also evaluates human
impacts that include thermal pollution. Ward
(1985) focuses on the Southern Hemisphere to
consider controls on the thermal regime and
anthropogenic factors. Caissie (2006) over-
views water temperature modelling, natural and
human influences on thermal conditions and
implications for aquatic ecology. Most recently,
Webb et al. (2008) capture renewed interest by
evaluating significant advances in river and
stream temperature research since 1990. Nota-
bly, Webb et al. (2008) identify improving
understanding of: (1) thermal heterogeneity at
different spatial and temporal scales; and (2)
past and future trends as major issues for con-
temporary river temperature research. Such
understanding is required urgently by environ-
ment regulators as a first step in assessing how
climate changes will alter river systems and
interact with other pressures affecting ecologi-
cal status and societal use of flowing waters.
This article offers a different perspective
from other reviews. It aims to evaluate changes
in river water temperature for the United King-
dom (UK) over the 20th century and possible
changes over the 21st century by synthesizing
the peer-reviewed literature. Although the
emphasis is on the UK, pertinent research is
reviewed to contextualize UK-based informa-
tion. Throughout, the term ‘river temperature’
refers to the temperature of the water column
in the river channel. Riverbed temperature is not
considered in detail.
The remainder of this article is structured as
follows: state-of the-art understanding of pro-
cesses, controls and dynamics and drivers of
change are reviewed to provide a clear scientific
basis for evaluating changes (section II);
changes in UK river temperature over the 20th
century are evaluated with reference to the
wider international literature to assess what has
happened (section III); what may happen in the
future is speculated based on the very limited
number of predictive modelling studies of pos-
sible UK temperature over the 21st century with
findings contextualized by international research
(section IV); and conclusions and emergent pri-
ority knowledge gaps are outlined to provide
guidance on how to improve understanding of
past, contemporary and future projections of
river temperature change in the UK (section V).
II Understanding processes,
controls, dynamics and drivers
of change
1 Processes
River water temperature is controlled by dynamic
energy (heat) and hydrological fluxes at the
air-water and water-riverbed interfaces (Figure
1; Hannah et al., 2008a). Land and water manage-
ment impact on these drivers and, thus, modify
river thermal characteristics (Webb et al., 2008).
Hannah and Garner 69
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Rivers are hierarchical systems (Montgomery,
1999) and therefore for a specific point on a river,
water column temperature is determined initially
by the mix of water source contributions (surface/
shallow subsurface flows, groundwater, snow
melt, etc.) and subsequently the energy gained
or lost across the water surface and riverbed inter-
faces as the river flows downstream. Thus, spatial
and temporal variability in heat flux and hydrolo-
gical processes create heterogeneity in river tem-
perature at a range of scales.
Heat transfer within river systems is complex,
occurring through a combination of radiation,
conduction, convection and advection (Webb
and Zhang, 1997). These energy exchanges add
and remove heat to and from the river. Inputs
may occur by incident short-wave (solar) and
longwave (downward atmospheric) radiation,
condensation, friction at the channel bed and
banks, and chemical and biological processes.
Losses may include reflection of solar radiation,
emission of longwave (back) radiation and
Figure 1. A schematic representation of the energy and hydrological fluxes controlling river water
temperature.
Source: After Hannah et al. (2008b)
70 Progress in Physical Geography 39(1)
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evaporation. Sensible heat and water column-bed
energy transfers may cause gains or losses. In
addition to these exchanges, energy may be
advected by: in/out-flowing channel discharge,
hyporheic exchange, groundwater up/down-
welling, tributary inflows and precipitation.
These heat fluxes may be related together using
a heat budget (or energy balance) to give the total
energy available to heat or cool river water (Q
n
)
at a given point:
Q
n
¼ Q
a
þ Q

þ Q
e
þ Q
h
þ Q
bhf
þ Q
f
ð1Þ
where Q
a
is advected heat due to groundwater
discharge, hyporheic exchange, precipitation,
and the longitudinal advective heat flux, Q* is
net radiation, Q
e
is latent heat, Q
h
is sensible
heat, Q
bhf
is bed heat exchange and Q
f
is friction
at the bed and banks.
Changes in water temperature (dTw)maybe
calculated within either an Eularian (e.g. Caissie
et al., 2007; Hebert et al., 2011) or a Lagrangian
framework (e.g. Gooseff et al., 2005; Leach and
Moore, 2011; MacDonald et al., 2014; Ruther-
ford et al., 2004). Within the Eularian frame-
work, dTw is calculated as a function of time
(t) (equation 2) using a reference system (i) that
is fixed in space and through which water flows:
dT
w
dt
¼
Q
nði;tÞ
rCD
ði;tÞ
ð2Þ
where r is density of water, C is specific heat
capacity of water and D is river depth. Within
the Lagrangian framework dTw is calculated
as a function of space (equation 3) using a refer-
ence system that moves with the water:
dT
w
dx
¼
W
ðiÞ
Q
nði;tÞ

C  F
ði;tÞ
ð3Þ
where W is width of the stream surface and F is
river discharge.
The ene rgy balance is a useful tool for
analysis of river temperature processes
(e.g. Hannah et al., 2008b; Leach and Moore,
2010; Garner et al., 2014; MacDonald et al.,
2014) and for river temperature prediction (e.g.
Gooseff et al., 2005; Westhoff et al., 2007; Leach
and Moore, 2011; MacDonald et al., 2014). For
the UK, river energy balance studies have been
conducted in the chalk streams (Webb and
Zhang, 19 97, 1999, 2004) a nd regulated riv-
ers (e.g. Evans et al., 1 998 ) of central and
southern England, and the Scottish Cairngorm
mountains (Hannah et al., 2004, 2008a; Garner
et al., 2014).
2 Controls and dynamics
River temperature controls are multivariate
and nested at macro-, meso- a nd micro-
scales (Fig ure 2) (Webb, 1996). Climate
drives the thermal regime in rive rs and is thus
the f irst-order control on regional patterns in
the magnitude and timing of seasonal
dynamics (Ward, 198 5; Garner et al., 2013).
Basin-wide characteristics are second-order
controls; water sources, basin aspect and pre-
cipitation regime may moderate the influence
of climate and thus modify the timing and
magnitude of subseasonal water temperature
dynamics. Reach-specific controls, which
interact with the water column as it moves
through the catchment, such as topographic
and riparian shading, hypo rheic exchan ges
and localized groundwater contributions, may
further moderate the inf luence of climate on
water temp erature dynamics. Thus, the cumu-
lative effect of contr ols at each s cale pro-
duces river temperature dynamics at a given
site and determines its sensitivity to climate
(Figure 2).
The l arge number of poten tial controls on
river temperature means that it is difficult to
disentangle their multivariate influence on
energy exchange, hydrological processes and,
ultimately, water temperature (Figure 2). How-
ever, water is typically cooler and less variable
at subse asonal scales where runoff is sourced
predominantly from groundwater (Figure 3)
(Erickson and Stef an, 2000; Kelleher et al.,
Hannah and Garner 71
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This review provides a new perspective on the topic by evaluating changes in river water temperature for the UK over the 20th century and possible changes over the 21st century.